Flow control valves, in particular in the form of electromagnetic directional flow control valves, have the function of setting the volume flow to a constant value independently of the pressure differential and the viscosity of a fluid. Depending on the design of the respective flow control valve, the volume flow can be controlled on the influent flow side or the return side of a connected hydraulic consumer. Flow control valves with adjustable volume flow can be implemented by proportional solenoids as the actuators such that a proportional solenoid can produce a parallel shift of the characteristic of the valve concerned.
In this context, the volume flow is adjusted continuously by the proportional solenoid that is actuated by an electronic power amplifier. The proportional valves can be position controlled or force controlled. Usually, a control piston, designed as a sliding piston, acts, subject to the action of the magnetic force of the proportional solenoid, against a compression spring such that an orifice cross section is correspondingly enlarged or decreased. The pressure independence of the volume flow is achieved by a differential pressure valve (pressure compensator) that provides a constant pressure differential at the metering orifice and that is normally downstream of this metering orifice.
A directional valve having this function is disclosed DE 196 04 317 A1. This valve has a hollow cone-shaped valve seat that forms a valve opening range with a valve element. In this case, the valve element has a spherical segmental section. The radius of the spherical segmental section and the opening angle of the hollow cone-shaped valve seat are established such that a sealing region is formed, when the valve element sits on the valve seat. The valve element is guided in a movable manner in a valve body, with the valve body having a corresponding inner circumferential surface that has a more or less uniform diameter over the entire length of the valve element.
In particular, such flow control valves of a seat-type design have at least two problem areas. First, the power demand for the proportional solenoid is relatively high. Second, such valves are difficult to actuate with a certain degree of precision in the extreme opening ranges. The electromagnetic directional valve, disclosed in the document, has a flat characteristic, so that the solenoid drive force is evened out in relation to an identical actuating current over almost the entire range of the valve stroke. This feature is implemented by setting the solenoid drive force such that it has a flat characteristic to make the control of the valve opening degree easier. However, this feature has the drawback that the solenoid drive force is relatively high with respect to a change in a range, in which the current value is correspondingly large. This results from the relationship that the solenoid drive force is proportional to the square of the current value. Therefore, the magnitude of change in the solenoid drive force becomes larger in relation to the same magnitude of change in the current.
The flow rate of a fluid to be controlled with such valves tends to change abruptly in relation to a small change in the actuating current in a range, in which the opening degree of the electromagnetic proportional flow control valve is small. Such flow control valves of the seat-type design have the problem that it is difficult to achieve an accurate flow rate control in a range with a low flow rate, at which the flow rate to be controlled is low.
In contrast, the prior art flow control valves of the sliding valve type design generally have a non-minimized power demand and/or a non-minimized size of the proportional solenoid.